Field of the Invention
The present invention relates to a positive electrode active material for a lithium ion secondary battery, and a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery comprising the same.
Background Art
Lithium ion secondary batteries have higher energy density and smaller memory effect than those of other secondary batteries, such as nickel-hydrogen rechargeable batteries and nickel-cadmium rechargeable batteries. For this reason, lithium ion secondary batteries have been increasingly used in a broader range of applications from small-sized power supplies for mobile electronic apparatuses and electric apparatuses for household use to middle- or large-sized power supplies such as stationary power supplies for electricity storage units, uninterruptible power supply units, and power leveling units and driving power supplies for boats and ships, rail cars, hybrid vehicles, and electric vehicles, and a further improvement in battery performance has been demanded. In particular, high energy density is required for lithium ion secondary batteries developed as middle- or large-sized power supplies so as to attain high capacity at low volume.
To meet such a demand, LiMO2 (where M represents an element such as Ni, Co, or Mn) positive electrode active materials having an α-NaFeO2 layer structure have been extensively developed because of their high charge-discharge capacity. Unfortunately, layered positive electrode active materials having a large content of Ni reduce charge-discharge cyclability, particularly output properties with cycles.
To solve these problems, techniques are proposed to improve the charge-discharge cyclability of the layered positive electrode active materials through a reduction in the amount of impurities present on surfaces of particles of the positive electrode active materials. For example, JP Patent Publication (Kokai) No. 2010-155775 discloses a positive electrode active material wherein powdery lithium composite compound particles represented by Li1−x+Ni1−y−zCoyMzO2 (where M is at least one or more of B and Al; −0.02≤x≤0.02, 0≤y≤0.20, and 0<z≤0.10) are prepared using a technique of washing particles with water to remove impurities, and an ionic strength ratio A (LiO−/NiO2−) is 0.3 or less and an ionic strength ratio B (Li3CO3+/Ni+) is 20 or less in analysis of the surfaces of the powdery lithium composite compound particles with a time-of-flight secondary ion mass spectrometer. In addition to this related art, JP Patent Publication (Kokai) No. 2013-026199 also discloses another technique of washing particles with water.
Other techniques are proposed in which the output properties of layered positive electrode active materials are enhanced by adjusting the voidages or open porosities of the positive electrode active materials. For example, JP Patent Publication (Kokai) No. 2014-67546 discloses a positive electrode active material comprising a secondary particle comprising primary particles having an average particle diameter of 0.01 μm or more and 5 μm or less, wherein the voidage is 3% or more and 30% or less and the open porosity is 70% or more.